1. Field of the Invention
The present invention generally relates to a method (and resulting lithographic mask or mold structure) to reduce the cost of fabricating a lithographic mask (or mold) by spatially segmenting the process to employ one or more independently verified multi-use imprint sub masks.
2. Description of the Related Art
Hereinbelow, the term “mask” is used to refer to an optical lithographic mask, and “mold” is used to refer to an imprint lithographic mold.
Present methods of mask making typically create (e.g., write) an entire mask (e.g., which typically contains at least one chip, but possibly as many as 4, 8 or 20 chips on a mask), and then verify the entire mask. However, as ground rules evolve to smaller and smaller dimensions (and thus as chips become smaller and smaller), this process (and the tools needed to produce the chip) becomes exponentially more expensive.
The construction of a mask is usually performed using e-beam lithography. Once constructed, the mask is then inspected and, if necessary, repaired. The mask write time and subsequent inspection is by far the longest part of the process. For example, a critical mask (e.g., using optical phase correction or OPC) may take 24-48 hours to print using the e-beam tool, but may take days to inspect. Hence, the inspection process (which may include some repair time of the defects found during the inspection) takes relatively the most time.
A reason for the write time and inspection time being so lengthy is that there is a combinatorial explosion in the number of features, in that as each feature becomes smaller the number of features which occur on a given mask increases exponentially. While this may be advantageous for the consumer since they obtain a product with much functionality at presumably less cost, fabrication becomes much more difficult since each feature must be checked.
That is, the features must be perfect since the mask will be used to replicate potentially millions of chips. Additionally, the mask cannot be inspected with an ordinary microscope, but instead a scanning electron microscope (SEM) must be used which is very expensive and time consuming. Additionally, traditional mask making employs photolithography or e-beam techniques to write the pattern in photoresist which is subsequently developed and etched.
The traditional mask making process itself introduces a multitude of distortions and defects that must be individually corrected and inspected. The serial printing and inspection of lithographic masks (or molds) is expensive and time-consuming. A phase shift mask might cost $150,000 and may require a month to fabricate and inspect.
Additionally, it is often observed that photolithographic mask features are typically 4 times (e.g., 4×) the final feature ground rule dimensions, and therefore require less precision. The use of phase shift features in optical masks dramatically impacts this observation by introducing additional precision requirements and topography that drive the tolerances to 1×.